1. Field of the Invention
The present invention relates to a voice presence/absence discriminator which discriminates between the presence and absence of an input voice signal in a voice analysis/synthesis system. More particularly, the invention concerns a voice presence/absence discriminator which, in a digital mobile radio communication system which transmits and receives coded data obtained by analyzing the voice signal in units of a frame per prescribed time period, is applied to a VOX (Voice-Operated Transmitter) as a low power consumption control for increasing the continuous use time length in a mobile station.
2. Description of Related Art
Conventionally, it has been usual that in a mobile station used in a mobile radio communication network the continuous conversation time is limited due to the limited capacity of the battery as a power source and therefore, an effort is made to extend the continuous conversation time by contracting various kinds of low power consumption controls.
As one of the various relevant techniques for such low power consumption control, there is known a VOX (Voice Operated Transmitter) control which, utilizing the fact that the continuous occurrences of speaking voices in a telephone conversation is small, stops the operation of the transmission circuit when no voice is generated, for example, when listening to the other party's speech, to thereby control the power consumption and increase the telephone conversation time.
In order to realize such VOX control, it is necessary to accurately discriminate between the presence and absence of a voice signal to be transmitted. As a device meeting such a requirement, Japanese Unexamined Patent Publication No. Hei 5-323996 discloses a voice presence/absence discriminator which is used in a transmitter equipped with a voice encoder that performs high-efficiency encoding of a voice signal in units of a frame per prescribed time length, and which discriminates between the presence and absence of the voice in units of a frame.
In this voice presence/absence discriminator, during the voice presence period in which the voice signals are being transmitted, discrimination between the presence and absence of the voice signal in the frame is performed by the use of the frame average power obtained every frame and parameters obtained in the voice encoder in the process of high-efficiency encoding of the voice signal which indicate the characteristics of the voice and, on the other hand, during the voice absence period in which the transmission of the voices is stopped, discrimination is performed between the presence and absence of the voice signals in the frame only on the basis of the frame average power because the voice encoder is also stopped for the purpose of reducing the power consumption.
However, in this device, since the presence and absence of the voices during the voice absence period is discriminated using only the average power of the frame, there is a problem that when a voice signal is beginning to be generated, the leading portion of the voice tends to be disconnected with the result that the naturalness of the voice is impaired.
That is, since the size of the frame used in this type of device is usually as long as 20 ms or so, when an utterance starts from the latter half of the frame, the average power of the frame that includes the leading portion of the voice becomes small and therefore it is likely that this frame is erroneously determined to be a voice absence frame. As a result of this, the disconnection of the leading portion of the voice takes place.
Also, when in order to prevent the occurrence of such disconnection the threshold value for discrimination between the presence and absence of the voices in the frame is set to a small value, the discrimination is likely to receive influences such as that of the fluctuation of the background noise, with the result that the erroneous operation of determining a voice absence frame to be a voice presence frame occurs frequently. As a result of this, there is the problem that it is impossible to execute the VOX control effectively.